The control of fetal lung surfactant production is being studied by assessment of its principal phospholipid components, phosphatidylcholine (PC) and phosphatidylglycerol (PG). We have developed a fetal rat lung organ culture system capable of synthesizing PC in increasing amounts during a five day culture period, closely parallelling rates of PC biosynthesis in vivo during comparable gestational ages. Insulin was found to stimulate PC and PG biosynthesis and the specific activity of the enzyme choline kinase. Morphine sulfate was found to inhibit the biosynthesis of PC in fetal lung organ culture.